Muscle history, fusimotor activity and the human stretch reflex

Abstract

1. The previous history of contraction and length changes of a muscle influences the size of the stretch reflex and H reflex. Here we ask, is this dependence due to changes in mechanical properties of extrafusal fibres, intrafusal fibres of spindles, or both? 2. The soleus muscle of human subjects was conditioned using either a voluntary contraction or a contraction evoked by low-strength electrical stimulation, in the range 0-25 % of maximum. Following conditioning, reflexes were increased by more than twofold above the no-contraction value by a voluntary contraction of 5 % of maximum, or more, but not by electrical stimulation which presumably did not contract the intrafusal fibres of spindles. 3. When the muscle was conditioned with a contraction at a length shorter than the test length, rather than at the test length, a depressing effect on reflexes was attributed to both the burst of impulses generated in spindles when the muscle was stretched back to the test length and to a reduced stretch sensitivity of muscle spindles. 4. The experiments demonstrate the importance of keeping the muscle and its spindles in a defined mechanical state when measuring reflexes. They also point to the powerful facilitating influences of conditioning muscle contractions provided they recruit the intrafusal fibres of spindles.

Figure 1. Conditioning and testing of human soleus muscle

The three methods of muscle conditioning are shown, in diagrammatic form, at the top. After stretch of the muscle (hold-long), shortening (hold-short), or no length change (hold-test), a fusimotor-strength contraction is given (filled bar). After the contraction the muscle is kept at that length for several seconds and is then returned to the test length where the test stimulus is given (arrow). The actual conditioning sequences employed in this study are represented by the sample traces shown at the bottom. For each trial the muscle was first pre-conditioned by being stretched, represented by 30 deg ankle rotation in a dorsiflexing direction from the 110 deg test position. At the longer length subjects carried out a 2 s contraction at a strength of 25% MVC. They were then instructed to completely relax while the muscle was held at that length for a further 5 s before being returned to the test length (hold-test) or to a length corresponding to 30 deg plantarflexion from the test position (hold-short). Three seconds after moving to the new position a conditioning contraction was given. The contraction was either voluntary or evoked by percutaneous electrical stimulation. Contraction strengths were adjusted to correspond to 0, 5, 10 and 25% MVC for the voluntary contraction, and 0, 5 and 10% for the electrical stimulation. Contraction strengths were measured as rectified, smoothed EMG (not shown). After the conditioning contraction subjects were again asked to fully relax while the muscle was returned to the test length 5 s later (hold-short) or kept at the test length (hold-test) and then the test stimulus was given to elicit a stretch reflex or H reflex. The top and second traces show ankle torque and soleus EMG, respectively, during pre-conditioning and hold-test conditioning, followed by the test reflex. The third and fourth traces show torque and EMG, respectively, during hold-short conditioning and test reflex. The bottom trace shows the ankle angle. The record of ankle angle is shown as a dashed line for hold-short conditioning and as a continuous line for hold-test conditioning. The reflex EMG in response to the tendon tap is shown on an expanded time scale.

Figure 2. Sample records of effects on the reflex, after pre-conditioning, of a voluntary contraction of 10% MVC (upper panel) or a 10% contraction evoked by percutaneous electrical stimulation (lower panel)

In each panel, the upper trace shows the raw EMG recorded in soleus, the second trace the rectified smoothed EMG record and the bottom trace, torque. Electrical stimulation leads to large artifacts which obscure the EMG record. The reflex after electrical stimulation is smaller than after a voluntary contraction.

Figure 3. Size of stretch reflex after two forms of muscle conditioning

Size of stretch reflex (means ± s.e.m.) normalized with respect to its maximal value, plotted against the size of the conditioning contraction. Circles, values obtained when a voluntary conditioning contraction was used; squares, contraction evoked by electrical stimulation. Open symbols, values measured after hold-test conditioning; filled symbols, values measured after hold-short conditioning.

Figure 4. Time course of conditioning effects on stretch reflex and H reflex

Upper panel: normalized stretch reflexes (means ±s.e.m.) measured after a 10% MVC conditioning contraction plotted against conditioning-test interval. ^, hold-test conditioning; •, hold-short conditioning. Lower panel: H reflexes, elicited using a stimulus strength of 25% of maximal motor response, measured after a 10% MVC conditioning contraction. ^, hold-test conditioning; •, hold-short conditioning.